1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and more particularly to a contact or a proximity type of charger and a cleaning device included in the image forming apparatus.
2. Description of the Background Art
Generally, an electrophotographic image forming apparatus includes various charging means, e.g., one for uniformly charging the surface of a photoconductive element or image carrier before the formation of a latent image, one for quenching the charged surface of the image carrier after image transfer, and one for charging a sheet or recording medium conveyed to an image transfer position. Such charging means have customarily been implemented by a corona discharge type of charging system. In this type of charging system, a charge wire is positioned in the vicinity of a body to be charged and applied with a high voltage, so that corona discharge occurs between the charge wire and the above body for thereby charging the body.
Corona discharge, however, produces ozone, NOx (nitrogen oxides) and other discharge products that are apt to form a nitric acid film or a nitrate film having adverse influence on an image on the surface of the image carrier. In light of this, a contact or a proximity type of charging system is extensively used today because it produces a minimum of discharge products and needs only a minimum of voltage.
In the contact or the proximity type of charging system, a roller, brush, blade or similar charging member contacts or adjoins a photoconductive element or similar body to be charged and applied with a voltage to thereby charge the surface of the above body. This type of charging system successfully reduces the size of the charger while having the advantages mentioned above.
However, a problem with the contact or the proximity type of charging system is that discharge occurs toward the body to be charged either directly or via a small gap, resulting in irregular discharge and therefore irregular charging. To solve this problem, Japanese Patent Laid-Open Publication No. 5-150564, for example, discloses a charging system configured to charge a body by applying an AC-biased DC voltage to a charging member. This system, using an AC-biased DC voltage, applies a voltage far higher than a breakdown voltage to the above body instantaneously and continuously, allowing discharge to easily occur. However, this system brings about another problem that the discharge chemically deteriorates the surface of the body charged. Such chemical deterioration, e.g., shaving of the film thickness of the photoconductive layer occurs even when mechanical rubbing is absent.
More specifically, the chemical deterioration of the surface of, e.g., the photoconductive element includes a decrease in molecular weight ascribable to the cut-off of the molecule chains of polycarbonate resin, which constitute the photoconductive layer, caused by ozone, active oxygen and charge particles hitting against the above surface, a decrease in the degree of entanglement of polymer chains, and evaporation of polycarbonate resin. Such chemical deterioration reduces the thickness of a charge transport layer (CTL) positioned on the surface of the photoconductive element little by little, causing inorganic fine grains contained in the CTL to separate and part. If the inorganic fine grains thus parted deposit on a cleaning member or similar member contacting the photoconductive element, then they constitute abrasive grains and therefore cause the surface of the element to locally wear, e.g., shaves off the surface in the form of stripes.
The deterioration of the surface of the photoconductive element ascribable to discharge is considered to be brought about by the energy of particles produced by discharge and is therefore considered to occur even when a material other than polycarbonate is used for the photoconductive element. Particularly, the AC-biased DC voltage generates discharge having greater energy than a DC voltage, aggravating the deterioration.
Further, when the shave-off of the photoconductive layer proceeds, it is more likely that the charge potential of the photoconductive element drops, the photoconductive element is deteriorated, background is contaminated due to, e.g., scratches formed on the surface of the element, image density decreases, and image quality is lowered. Protecting the surface of the photoconductive element is extremely important when consideration is given to an increasing demand for high image quality.
It is a common practice to protect the surface of the photoconductive element from deterioration by, e.g., coating the above surface with amorphous silicone carbide to thereby enhance wear resistance or by dispersing alumina or similar inorganic substance in the CTL of an organic photoconductive layer for the same purpose, as taught in, e.g., Japanese Patent Laid-Open Publication Nos. 2002-207308 and 2002-229227. Such schemes, however, are not always successful to obviate the chemical deterioration ascribable to proximity type of charging although they may improve wear resistance. This is because a decrease in the film thickness of the photoconductive element has heretofore been attributed to mechanical wear ascribable to a contact member, but not to discharge to occur in the event of charging.
On the other hand, Japanese Patent Laid-Open Publication Nos. 2002-55580, 2002-244487, 2002-244516 and 2002-156877 each propose an image forming apparatus including means for coating zinc stearate on the surface of a photoconductive element. However, the means taught in these documents are configured to reduce the coefficient of friction of the surface in order to obviate toner filming, toner melting and defective cleaning. The above documents therefore do not address to the protection of the surface of a photoconductive element from deterioration ascribable to discharge. Moreover, zinc stearate, simply coated to reduce the coefficient of friction, cannot always obviate deterioration ascribable to discharge.
More specifically, the schemes taught in Laid-Open Publication Nos. 2002-55580 and 2002-244487 mentioned above are likely to fail to protect, in the contact or the proximity type of charging system, the surface of the photoconductive element from chemical deterioration conspicuous with discharge caused by an AC voltage (AC discharge hereinafter). This is because the condition in which zinc stearate should exit for obviating the chemical deterioration and the condition in which it should exist for reducing the coefficient of friction are different from each other. Laid-Open Publication Nos. 2002-244516 and 2002-156877 do not show or describe means for coping with the chemical deterioration at all, not to speak of an adequate condition in which a protection substance should exist in a discharge zone.
In an image forming apparatus, a charge voltage or similar charging condition is sometimes varied in accordance with temperature, humidity or similar environmental condition for thereby implementing optimum image forming operation. More specifically, the rate at which the chemical deterioration of the photoconductive element proceeds is presumably dependent on the charging condition. It follows that if a condition of presence of the protection substance on a body to be charged capable of adequately obviating the chemical deterioration can be found, it is possible to further enhance the durability of the above body.